1. Field of the Invention
This invention relates generally to a mooring system for floating storage vessels and particularly to a single point mooring system which includes a fluid flow path from a subsea structure to a vessel for mooring the vessel and loading hydrocarbons thereon or discharging hydrocarbons from the vessel to the subsea structure.
2. Description of the Prior Art
European patent application publication EP 0 796 784 A1 shows a gimbal or swivel mounted on a base at the sea floor for connection of separate mooring lines and a flexible hose (riser). A rotation collar connects the mooring line to the base.
International patent application publication number WO 99/57413 discloses a composite hybrid riser having a central tension member surrounded by a plurality of fluid transmitting tubes.
U.S. Pat. No. 5,927,224 shows a turret moored system anchored by dual function riser/mooring lines. Each hybrid line includes an outer cylindrical shell which serves as a tension member. One or more conduits inside the outer shell serve as fluid conduits between the vessel and a subsea manifold.
3. Identification of Objects of the Invention
A principal object of the invention is to provide an improved single point mooring system for mooring and fluid transfer between a submerged structure and a floating body which utilizes a buoyant hybrid fluid conductor/tension member as the anchor leg and the fluid flow path.
Another object of the invention is to provide a disconnectable mooring system by which a vessel or other floating body is moored about a single point by means of a buoyant hybrid riser tension member arrangement.
Another object of the invention is to provide a buoyant hybrid fluid conductor/tension member having a fluid conduction path, a tension member and buoyancy material along the length of the member, so that a single member, having a length that reaches from a subsea structure to a vessel, serves as a conduit for the transfer of hydrocarbon fluids and serves as a single anchor leg with restoring force.
Another object of the invention is to provide a buoyant hybrid fluid conductor/tension member which is rigidly connected to a submerged structure and to a floating body where the member twists as the vessel weathervanes about the submerged structure.
Still another object of the invention is to provide a buoyant hybrid fluid conductor/tension member which alternatively includes a load transferring rotatable fluid connection between a submerged structure and a floating member.
The objects identified above, along with other features and advantages of the invention are incorporated in a mooring system where a buoyant hybrid riser/tension arrangement (BHRT) moors and fluidly couples a floating body on the sea surface to a subsea structure such as pipeline end manifold (PLEM) at the sea floor, a submerged tower, a submerged TLP structure or a submerged buoy. The floating body may be a dedicated shuttle tank, shuttle tanker of opportunity or a Floating Storage and Offloading vessel (FSO) or a Floating Production Storage and Offloading vessel (FPSO). The BHRT includes one or more conduits, buoyancy members and tension members. The tension members may be the walls of tubular conduits or a separate tension device such as a stranded wire cable. The conduits establish one or more fluid flow paths between the submerged structure and the floating body. The tension members and buoyancy members allow the floating body to weathervane about the submerged structure, while keeping the floating body on station, utilizing tensile anchoring and buoyancy of the BHRT, to produce a soft restoring force.
A coupling of the BHRT to the submerged structure allows angular, but not torsional displacement of the lower end of the BHRT. In one embodiment of the BHRT lower end, localized flexing is provided in the separate conduits via bend stiffeners. At the BHRT upper end, several embodiments of a BHRT/Floating body coupling are provided. In a first embodiment, a rigid connection is established between a male coupler at the upper end of the BHRT and female coupler on the floating body. All torsional displacement occurs along the length of the BHRT between the subsea structure and the floating body. In a second embodiment, the upper end of the BHRT includes a riser end buoy with a mating surface that couples with a female receptacle mounted on a bearing assembly on the floating body. On top of the female receptacle is an ESD valve block and a swivel (in a first embodiment) or a manifold block (in an alternative embodiment).
In another embodiment, a load transferring rotatable fluid connection or swivel is provided in the BHRT between the submerged structure and a floating member.